Excess estrogenic substances in rodent diets may influence growth, diseases, tumor rates, reproductive performance, and the results of comparative estrogenic and carcinogenesis studies. A standard method for conducting the mouse bioassay for the detection of estrogenic activity in rodent diets was published. We have shown that rodent diets differ significantly in estrogenic activity. In addition, we have reported that some rodent diets contain high concentrations of the phytoestrogens daidzein and genistein equivalent to 4.3 ppb of DES activity and that these phytoestrogens may alter the results of comparative estrogenic and carcinogenicity studies. We concluded that an open formula standardized diet ideally free of estrogenic substances, is essential when performing comparative estrogenic or carcinogenicity studies. We have shown that natural ingredient diets can be formulated to contain less than detectable levels of the phytoestrogens daidzein and genistein by omitting the soybean and alfalfa meals. - phytoestrogens, plant hormones, isoflavones, genistein, daidzein, ligans